The present invention relates to a high NA (numerical aperture) objective lens installed in an optical pick-up of an optical disc apparatus for a digital versatile disc (DVD), a magneto-optical disc (MO) or the like. Particularly, the present invention relates to the objective lens that has a diffractive grating formed on a surface of a refractive lens.
An optical pick-up of an optical disc apparatus employs, in general, a single-piece plastic objective lens having both aspherical surfaces. Plastic lenses have advantages since the weight is relatively light, and the lenses can be formed relatively easily by an injection molding process.
However, the plastic lens has some disadvantages as follows. The major disadvantage is significant performance change depending on the temperature. It is because, the plastic lens has higher sensitivity to temperature in terms of the refractive index, and larger heat expansion property than a glass lens. For instance, when temperature rises, the refractive index of the plastic lens decreases, then spherical aberration changes to be overcorrected, and consequently, wave front aberration increases. When the wave front aberration exceeds the predetermined permissible level, the objective lens cannot form a sufficiently small beam spot on an optical disc. The sensitivity, which is a ratio of the refractive index variation with respect to a temperature change, is approximately -10.times.10.sup.-5 /.degree. C.
FIG. 27 is a graph showing a change of wave front aberration (unit:.lambda., wavelength) with respect to the NA for a plastic lens (focal length: 3.0 mm at wavelength 650 nm), when the temperature rises by 40 degrees and the refractive index is changed by -400.times.10.sup.-5. As shown in the graph, the change of the wave front aberration when the temperature is changed is substantially proportional to the fourth power of NA.
The NA of an objective lens for a CD apparatus is approximately 0.45 and the permissible level of the wave front aberration is approximately 0.04.lambda.. This allows temperature variation in a range of 90 degrees. For example, the range is 30.+-.45.degree. C. Since the temperature range allowing the permissible level of the wave front aberration is sufficiently large, the deterioration of the wave front aberration due to temperature variation is virtually out of question in the CD apparatus.
On the other hand, the NA of an objective lens for the DVD apparatus is about 0.60, that for the MO disc apparatus is about 0.65 and the permissible level of the wave front aberration is approximately 0.03.lambda.. In this case, when the temperature changes by 40 or 50 degrees, the wave front aberration exceeds the permissible level, which may cause a problem in reading and/or writing information.